Silicon carbide (SiC) is a semiconductor material with properties highly suitable for high power, high frequency, and high temperature applications. Its wide bandgap make it suitable for high temperature applications and also as a substrate for optoelectronic applications where blue and UV light is desired. Many applications require a very high quality SiC crystal to minimize device defects and failures. Such high quality silicon carbide is difficult to produce in an efficient manner. Technical obstacles have remained that have inhibited the widespread use of silicon carbide. Reduction in defects must be achieved to realize the full potential of silicon carbide in the electronics industry. Silicon carbide substrates are typically perforated by tiny holes called micropipes at a density of 15-30 mp/cm2 on good material. The diameter is also small, only 3″ on commercially available material. To make a viable business, low cost, high quality, four-inch (minimum) wafers must be produced.
The standard way of growing SiC is by seeded sublimation growth. A graphite crucible is filled with SiC powder and a SiC single crystal seed is attached to the lid of the crucible, which is then sealed. The system is heated to temperatures above 2000° C. where SiC sublimes. Temperatures must be quite high to make sure the SiC powder sublimes appreciably. If a thermal gradient is applied such that the seed is colder than the source material, transport will take place from the source to the seed. If the pressure is lowered to a few torr, the material transport is enhanced. Unfortunately, the method has some drawbacks. Micropipe density is significant. Purity is also often a problem. Due to the way the thermodynamics work for the sublimation, the growth is generally rich in silicon (Si) at the beginning, with diminishing amount of Si at the end of the growth. This has severe implications on the yield of semi-insulating wafers since the material will be n-type at the start of the growth and p-type at the end.
The length of the grown crystals, commonly called boules, is also limited to the amount of silicon carbide source material in the system. Typical boule lengths are 20-25 mm.
High Temperature Chemical Vapor Deposition (HTCVD) can also be used to produce silicon carbide crystals. Gases carrying the Si and C needed for the growth replace a powder source material. In the entrance region, silane and ethylene react and nucleate in the gas phase to form micro-particles of SixCy. Carbon is sometimes omitted and is then grabbed from graphite walls of the crucible. Micro-particles of SiC are formed and are moved into a main chamber where the temperature is much higher. Here the particles sublime and move towards a colder substrate through the aid of a carrier gas and the thermal gradient.
Unlike seeded sublimation growth, HTCVD is an open system, and is therefore run at significantly higher pressures to avoid problems with sublimation (evaporation) of the sample.
Material properties of HTCVD grown silicon carbide are usually much better than that of the sublimation grown crystals, however, the defect density could still use improvement, growth rates are low (<1 mm/hr), and temperatures are high which stresses the crucible and insulation materials making the system drift.
Accordingly, there is a need for a lower temperature silicon carbide growth method that produces high quality crystals with a minimum amount of defects, and at higher growth rates (greater than 1 mm/hr).